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Abstract Soil biogenic components are subject to continuous sequestration, and export from soils into the surrounding air and water environments. However, the processes involving the stability or lability of their mineral states remain still unclear. To assess these issues, we have measured various biogenic components in a number of agricultural, forest, grassland, and deep soils, as well as desert sands from Inner Mongolia, both in the solid state and liquid extracts. The contents of soil organic carbon (SOC) and soil total nitrogen (STN) were higher in soils than in sands, whilst those of soil total sulfur (STS) and inorganic carbon were higher in sands and deeper soils. The significant positive correlations found between STS and SOC, and STN, and their significant negative correlations with pH and δ13C-SOC in all soils suggest a pH-dependent sequestration of C, N, and S. The decreased stability of organo-mineral complexes at acidic pH, resulting from the acidification of humic substance (HS) functionalities, leads to a higher availability of nutrients that facilitates the sequestration of soil organic matter (SOM). Conversely, an increase in pH enhances the stability of organo-mineral complexes by promoting negatively charged HS functionalities, which reduces the availability of nutrients and the sequestration of SOM. The δ13C-SOC enrichment in desert sands (-17.63 to -7.10‰) and its depleted values in soils (-24.9 to -18.8‰) suggest the occurrence of C sequestration in desert, via uptake of enriched atmospheric CO2 (-8.4‰). The fluorescence spectra of humic substance components and their molecular weights in sands were typically different from those of soils. The predominant relatively low molecular weight (MW) ( 25 − 15 kDa) of water-extracted (labile state: LS) components of all soils suggest, respectively, their involvement in organo-mineral complexes and for export into the surrounding environment. The quantities of LS and CS soil components differ significantly on dependence of soil characteristics, implying their corresponding lability or stability in soils. These findings will provide useful input for the management of the corresponding soil/sand ecosystems.

Abstract Ferric ions can bind strongly with dissolved organic matter (DOM), including humic acids (HA), fulvic acids (FA), and protein-like substances, whereas isolation of Fe-DOM precipitates (Fe-DOMP) and their biochemical characteristics remain unclear. In this work FeCl3 was used to isolate DOM components from various sources, including river, lake, soil, cow dung, and standard tryptophan and tyrosine, through precipitation at pH 7.5–8.5. The Fe-DOMP contribute to total DOM by approximately 38.6–93.8% of FA, 76.2% of HA and 25.0–30.4% of tryptophan and tyrosine, whilst fluorescence spectra allowed to monitor/discriminate the various DOM fractions in the samples. The relative intensity of the main infrared peaks such as 3406‒3383 cm−1 (aromatic OH), 1689‒1635 cm−1 (‒COOH), 1523–1504 cm−1 (amide) and 1176–1033 cm−1 (‒S=O) show either to decline or disappear in Fe‒DOMP. These results suggest the occurrence of Fe bonds with various functional groups of DOM, indicating the formation of π–d electron bonding systems of different strengths in Fe‒DOMP. The novel method used for isolation of Fe-DOMP shows promising in opening a new frontier both at laboratory and industrial purposes. Furthermore, results obtained may provide a better understanding of metal–organic complexes involved in the regulation of the long-term stabilization/sequestration of DOM in soils and waters.

Abstract Upon exposure to sunlight extracellular polymeric substances (EPS) were partially transformed into transparent exopolymer particles (TEP) and unstable flocs of different sizes without the addition of any precursors. Parallel factor (PARAFAC) modelling of the sample fluorescence spectra identified humic-like and protein-like or tyrosine-like components in both untreated and irradiated EPS samples. After 58 hours of solar irradiation, humic-like substances were entirely decomposed, while the regenerated protein-like substance from EPS was the key component in the irradiated samples. Degradation and reformation of EPS occurred which was confirmed by the results of size exclusion chromatography, dissolved organic carbon, total protein and total polysaccharide analyses. Irradiated EPS was composed of –COOH or C = O (amide I band) and –NH and –CN (amide II band), while Fourier transform infrared spectroscopy (FTIR) of TEP revealed more acidic –COOH and –C–O groups, indicating typical acidic protein-like TEP. The regenerated protein-like substances could form complexes with free metals originating from degraded EPS in irradiated samples, which could be responsible for the formation of TEP/floc in the aqueous media. These results suggest that TEP/floc formation from EPS could occur by a complexation mechanism between dissolved organic matter and metals, thereby causing ionic charge neutralisation upon sunlight exposure.